Modern surveillance systems incorporate various types of sensors for detecting persons within an area under surveillance. In some instances, the surveillance systems include a motion detector that uses radio frequency (RF) waves to sense motion of objects within the area under surveillance. In such cases, the motion detector transmits an RF wave and receives a reflected RF wave from the object. Based on when the reflected waves are received, a distance between the object and the motion detector can be determined. The motion detector may monitor the distance between the object and the motion detector over time and trigger a notification or alarm when motion is detected. In this way, the motion detector may detect the presence of a person, for example an intruder, in the area under surveillance.
However, in some cases, the motion detector may detect motion from other objects besides persons. For example, the motion detector may detect motion of insects, rodents, birds, and the like. As a consequence, false alarms may be triggered by the motion detector. Due to the nature of using reflected RF waves, motion detectors are highly sensitive to motion caused by close objects and less sensitive to objects further away. In addition, to detect persons at large distances, the motion detector must be sensitive to low amplitude reflections. As a consequence, motion detectors as currently known in the art balance long detection ranges with a quantity of false alarms.
FIG. 1 illustrates a graph of signal strength of the reflected RF wave versus distance to an object for a motion detector known in the art. The graph includes examples of signal strength for various objects detected by the motion detector over a detection range. In this example, the detected objects include a person, a rodent, and a bug. A threshold 120 determines when a controller triggers an alarm based on the signal strength. For example, when the signal strength is greater in amplitude than the threshold 120, a controller triggers an alarm. As illustrated, when the detected objects are near in distance to the motion detector (for example, under 4 feet), the reflected RF waves and associated signal strengths are high in amplitude. As a consequence, the person, the rodent, and the bug all trigger the alarm at close distances. This results in false alarms when the objects are at close range.